If you have a part inside a model, and a Highlight also inside that model, re-parenting the part to the workspace (or any other parent) will not un-highlight the part.
While my example is in a static studio environment, this issue still happens at run-time in games.
Two parts in a model with a highlight:
One part removed from the model, but still retaining a highlight:
Place file to reproduce:
HighlightBug.rbxl (56.2 KB)
Expected behavior
I expect that when a part is removed from a model with a highlight, the part does not retain the highlight and becomes un-highlighted